transport: add data_frame_budget to client Endpoint

[LukasKorba]

⚠ Blocked on hyperium/hyper#4164 — draft until it merges and releases.

Motivation

h2 0.4.18 added a client-side budget (client::Builder::data_frame_budget)
that guards a connection against floods of small DATA frames, closing it
with GOAWAY(ENHANCE_YOUR_CALM) once the budget is exhausted. This is a
correctness fix in h2 (see hyperium/h2 CHANGELOG.md 0.4.16-0.4.18), but
its default of 25,600 bytes can be exhausted by well-behaved peers: a
server-streaming RPC that emits one small message per frame -- one
HTTP/2 DATA frame per item in a long stream -- can trip the guard
partway through, even though both sides are individually within the
HTTP/2 spec.

tonic already exposes sibling h2 client settings on Endpoint
(max_frame_size, initial_stream_window_size, http2_header_table_size,
...), but had no way to raise this new budget, so callers hitting the
guard had no mitigation short of pinning h2 below 0.4.16.

This depends on hyperium/hyper exposing the setting on
client::conn::http2::Builder, which it does not yet as of hyper 1.11.0
(see the sibling expose-data-frame-budget branch on hyperium/hyper);
this change should land after that one merges and releases.

Solution

Add a new data_frame_budget field to Endpoint, mirroring the
max_frame_size pattern:

• Add data_frame_budget: Option<usize> field to Endpoint
• Add public pub fn data_frame_budget(self, budget: impl Into<Option<usize>>) -> Self builder method
• Wire the field through to hyper's Builder::data_frame_budget() in
  connection.rs
• Include an integration test verifying the setting is accepted and a
  basic RPC still completes end-to-end (h2's own test suite covers the
  budget's flood-detection behavior; this test only proves tonic's
  plumbing)

Evidence

hyper, cargo test -p tonic --all-features is green (212 tests incl. a new doctest), a new
integration test (tests/data_frame_budget.rs) proves the plumbing end-to-end, and against a
real public gRPC server that emits one small DATA frame per message (Zcash lightwalletd,
10,000-message range): default budget trips h2's too_many_data_frames guard on the released
stack; endpoint.data_frame_budget(256_000) completes the same range; data_frame_budget(1024)
fails immediately — the knob verifiably drives h2 through the whole chain. Motivating context:
zcash/lightwalletd#593.

[linux-foundation-easycla]

• ❌ - login: @LukasKorba / name: Lukas Korba. The commit (f6e72d5) is not authorized under a signed CLA. Please click here to be authorized. For further assistance with EasyCLA, please visit our EasyCLA portal and chat with our support bot.